- 初始状态 pending
- 执行了 resolve Promise状态变成 fulfilled
- 执行了 reject Promise状态变成 rejected
- 状态不可逆
- Promise中有throw =》 rejected
- then接收两个回调 成功和失败
- fulfilled 执行成功的回调 rejected执行失败的回调
- 存在定时器需要定时器执行结束后才执行then
- then函数是异步执行的微任务
- then 支持链式调用，下次then接收上次then的返回值
- all实现
class myPromise{
constructor(executor){
this.initValue()
this.initBind()
try {
executor(this.resolve,this.reject)
} catch (error) {
this.reject(error)
}
}
initValue() {
this.PromiseState = 'pending'
this.PromiseResult = null
this.onFulfilledCallback = []
this.onRejectedCallback = []
}
initBind() {
this.resolve = this.resolve.bind(this)
this.reject = this.reject.bind(this)
}
resolve(result) {
if (this.PromiseState !== 'pending') return
this.PromiseState = 'fulfilled'
this.PromiseResult = result
while (this.onFulfilledCallback.length) {
const { onFulfilled, resolve } = this.onFulfilledCallback.shift()
const result = onFulfilled(this.PromiseResult)
resolve(result)
}
}
reject(reason){
if (this.PromiseState !== 'pending') return
this.PromiseState = 'rejected'
this.PromiseResult = reason
while (this.onRejectedCallback.length) {
const { onRejected, reject } = this.onRejectedCallback.shift()
const resultErr = onRejected(this.PromiseResult)
reject(resultErr)
}
}
then(onFulfilled,onRejected){
onFulfilled = typeof onFulfilled === "function" ? onFulfilled : val => val
onRejected = typeof onRejected === "function" ? onRejected : val => val
var thenPromise = new myPromise((resolve, reject) => {
queueMicrotask(() => {
if (this.PromiseState === 'fulfilled') {
let result = onFulfilled(this.PromiseResult)
resolve(result)
}else if(this.PromiseState === 'rejected'){
let err = onRejected(this.PromiseResult)
reject(err)
}else if(this.PromiseState === 'pending') {
this.onFulfilledCallback.push({ onFulfilled:onFulfilled.bind(this), resolve: resolve.bind(this) })
this.onRejectedCallback.push({ onRejected: onRejected.bind(this), reject: reject.bind(this) })
}
})
})
return thenPromise
}
static all(promiseList) {
const result = []
let count = 0
return new myPromise((resolve, reject) => {
const addData = (index, value) => {
result[index] = value
count ++
if (count === promiseList.length) {
resolve(result)
}
}
for (const [index, promise] of promiseList.entries()) {
if (promise instanceof myPromise) {
promise.then(
res => { addData(index, res)},
err => reject(err)
)
}else{
addData(index, promise)
}
}
})
}
}
